1. Field of the Invention
The present invention relates to a drying apparatus for drying a surface of a planar target, such as a wafer, for use in a lithography system for projecting an image pattern onto the target. The present invention further relates to a lithography system comprising such a drying apparatus. Finally, the present invention further relates to a method for drying a planar target.
2. Description of the Related Art
In known lithography systems, the target, such as a wafer, to be patterned is subjected to incidence of photons or charged particles such as ions and electrons. So as to realize high precision patterning of the target, it is to be firmly bonded or connected to a target table (or wafer table) by means of which the target is moved relative to the source of incidence, at least if position measurement of said target is performed via said target table. It is preferably also to remain in position relative to said table during handling of the target, e.g. during insertion and removal of the target to and from the position where it is to be processed.
Various solutions exist for meeting the above requirements, e.g. by means of electromechanical clamping or clamping by capillary action using a fluid and causing the wafer undesirably to become wet. An example of such a clamping system utilising capillary clamping is for example known from International Application WO2009/011574.
In the known system a target is included in a lithography system by means of a target table and clamping means are present for clamping said target on said table. Said clamping means comprises a layer of stationary liquid, included at such thickness between target and target table that, provided the material of the liquid and of the respective contacting faces of the target and target table, a pressure drop arises. The stationary fluid may be for example water. After patterning, the target and target table are removed from the lithography system and unclamped. i.e. the target and target table are separated.
In the known clamping system, after unclamping the target needs to be dried before returning back to further processing, such as a coater/developer track. The target backside may be wet with liquid droplets (such as water droplets) after unclamping from the capillary clamping table. Liquid spots left on the wafer surface can cause oxidation that damages components on the wafer.
Besides the known clamping system, other stages of the lithography process such as coating, etching and cleaning may leave liquid on the target when the wafer is processed. In a lithography system, especially in a vacuum system, these liquid droplets inside system should be avoided as these may contaminate the wafer during further processing or interfere with the rest of the system. The wafer thus should be dried before it is further processed.
Since the target is to be processed in a lithography system, there are several requirements placed on the drying apparatus. The target should be dry enough, i.e. no liquid or droplets remain, since any remaining liquid or droplets may still negatively influence the lithography process.
Further, the drying apparatus should not add contamination to the target whilst drying, again to not negatively influence the lithography process. Since a target such as a wafer passes through the entire lithography system, any contamination present on the target will spread throughout the entire system.
After drying the target may need to be processed further. For some stages of processing, it is required that the temperature of the target is controlled, thus the drying apparatus should not raise or lower the temperature of the target in a manner that disrupts the lithography process
Due to limited space, which is also highly expensive, available in cleanrooms where lithography processes typically take place, the drying apparatus should preferably be of a small and compact size relative to the lithography system.
Several ways of drying are known in the art. A first known way of drying is spin drying, where the target or wafer is spun and the centrifugal force removes the liquid or droplets. Spin drying typically takes place at speeds of 2000 rpm. Ideally, wafers are spun off-axis e.g. spun around an axis that does not coincide with the center of the target. Disadvantages of this method are that spinning the wafer, especially off-axis, introduces unwanted vibrations. Also, off-axis spinning increases the footprint of the drying apparatus, which is not wanted. If on-axis drying in used, e.g. the wafer is spun around the center of the wafer, it was found that large residual droplets are left on the wafer.
Another know way of drying wafers is by using an “air knife”. In such systems, the wafer passes through an high speed gas flow (e.g. nitrogen, N2) to remove droplets by blowing them away from the surface of the target. Typical flow speeds for the gas are 50 m/s to 200 m/s. In this manner droplets are blown away, not evaporated. This type of drying is often employed in laboratories, and is not typically used in industrial lithography applications. A disadvantage is the use of an air knife place high aerodynamic forces are exerted on wafers. Typical wafers are of 300 mm size and can fail when exposed to the forces exerted by the air knife. Further, since the liquid is blown away and not evaporated, the liquid may contaminate other parts of the system.
Another known way of drying wafers which is commonly used in lithography processing is Marangoni drying. In this process, a wafer is vertically retracted from a liquid bath, typically water. IPA (isopropyl alcohol) vapour and heated N2 is made to flow over the wafer, causing a surface tension gradient in the liquid. This allows the liquid film to smoothly recede from the wafer since the presence of a gradient in surface tension will naturally cause the liquid to flow away from regions of low surface tension. The heated N2 flows across the wafer to evaporate the IPA. A typical Marangoni drying speed is 6 mm/s leading to a total drying time of 50 s for a 300 mm wafer. Disadvantages of Marangoni drying include the complexity and size of the drying system since the wafers have to be vertically pulled and the relatively low drying speed.
It has been suggested to combine Marangoni drying with an air knife, where the wafer is passed through an IPA saturated air knife. It is however not clear if the surface tension gradient may also occur in the turbulent aerodynamic conditions in the air knife. Also, residual condensed IPA is blown away by the air knife in an uncontrolled manned, and can cause contamination of the system.
Marangoni drying may further be combined with a form of spin drying (then called Rotagoni drying) where the IPA displaces the liquid on the wafer under centrifugal force, the IPA then evaporates. Besides the disadvantages outlined above under spin drying and Marangoni drying, this combined technique is normally only useful for the top wafer surface.
The use of wipers has also been suggested for drying wafers, it was found however that liquid drops may remain on the wafer using a wiper. The removed liquid remains on the wiper and may fall off, contaminating the system. Also, bottom of the wafer is touched by the wiper, which is undesirable and may lead to contamination of the wafer.
Applying heat is a known way of drying, but is relatively slow for wafer drying and may leave spots on the wafer. Conversely, freeze drying may be used, however the very low temperatures may have adverse effects on the wafer.